Digital color printing has gained popularity for producing personalized works, containing individual information as well as color content for brand identity, promotional and other informational material where color tends to add clarity. Digital color printing is quickly replacing processes where preprinted forms were used to supply non-variable color content followed by a secondary process that printed the variable content. There are numerous reasons for this trend including better overall economic value and a more effective end product.
As a result of this trend, the need for monitoring and identifying shifts or errors in the printed colors has become important. This is especially true where the colors printed are related to brand identity. In addition, it is common for these printing operations to occur on a number of different devices and in different physical locations. This creates the need to assure consistency within a print run (page to page), from print run to print run, from printer to printer and from location to location.
To address this need a system is required that can be incorporated inline with the printing process to scan, acquire and analyze the color content in the image. Such a system must be able to image web widths up to and possibly beyond 40-inches, and at speeds of 1000-feet per minute and beyond. Such a system must also deal with the wide gamut of color that can be produced with these types of printers.
The above stated speeds and web widths pose significant technical challenges to making inline, real-time, accurate and repeatable color measurements. For example, conventional spectrophotometers are limited to small viewing areas (typically 5 to 10 mm wide) and require the sample area to be stationary while the measurements are being made. It would require a large number of spectrophotometers to inspect the entire web width and this alone would still not address how to measure in real time. Such an approach would be physically and economically impractical.
Alternatively, using color electronic cameras to obtain an image and analyzing that image to measure the color content across the complete web introduces a new set of issues that need to be overcome. These issues include the following. 1) The impracticality of equally illuminating a wide web with the same concentration of each wavelength of light. 2) The inability to image a wide area without introducing significant optical chromatic aberrations. 3) The inability to image a wide area without introducing significant geometric distortions. 4) The inability to calibrate a camera based solution in a manner that provides accurate color measures over the complete printing gamut. This inability is due to the fact that the camera system is based on a three-channel wide-band filtering system versus the traditional multi-channel narrow-band technology of a typical spectrophotometer.
It is therefore apparent that a new approach to inline color monitoring is needed. The current patent application provides for a system that is relatively low-cost and removes variations in optical and mechanical parameters between different selected locations to determine in real-time the deviation of color at any location on a substrate and actively provide color quality control on any given page and amongst all printers tied to the system.